1. Field of the Invention
This invention relates to apparatus for testing valve actuators. More specifically, this invention relates to an apparatus for subjecting a valve actuator to dynamic and seating loads which simulate the loads an actuator would experience in opening and closing a butterfly-type valve in a fluid conduit.
2. Description of the Prior Art
In the art, butterfly valves are well known for controlling flow of a fluid through a conduit. Such valves open and close the flow of fluid by rotating a valve disc to a valve fully opened position with the disc parallel to the flow of fluid through the conduit. The valve is closed by rotating the disc 90.degree. to a valve fully closed position with the disc at right angles to the fluid flow path. Proper sealing of the valve is maintained by providing the disc with a resilient sealing edge which is received against a smooth valve body seat (usually fabricated from stainless steel) when the disc is in the fully closed position.
Commonly, such valves are operated by electric motor driven actuators which operate to rotate the disc 90.degree. between the valve fully opened and the valve fully closed position. While operating the valve, the actuators are subjected to varying torque loads. One such load is a dynamic torque load experienced by the actuator while rotating the disc from open to close and close to open. Another such load is a seating torque load experienced when the disc seal edge is forced against the valve seat while the valve is positioned at fully closed.
The dynamic load experienced by the actuator varies with the angular position of the disc within the conduit. For example, when the valve is at fully opened position with the disc in parallel to the fluid flow (conveniently referred to as 0.degree. position), little or no dynamic torque load is experienced. As the valve moves toward fully closed position (with the position of the valve measured as the lesser included angle between the disc and the disc opened position), the dynamic torque load increases to a maximum positive dynamic torque load. Typically, the maximum dynamic torque load occurs when the valve is between the 20.degree. and 40.degree. position with 30.degree. representing the most frequently occurring position at which maximum dynamic torque load is experienced. As the valve moves to about a 60.degree. position, the dynamic torque load decreases to zero. After the 60.degree. position, the dynamic torque load continues to decrease with a negative value. This negative torque load occurs by reason of the fluid flow urging the disc toward the valve seat. During this period of valve operation, the actuator must work to resist the forces of the fluid flow which would otherwise tend to force the disc to impact the valve seat with such force as might damage the seat.
When the valve proceeds to nearly the fully closed position (for example, at approximately an 80.degree. position), the actuator must work to drive the disc seal edge against the valve seat resulting in a suddenly increasing seating torque load which reaches a maximum positive value at the 90.degree. position (valve fully closed).
Often, it is desirable to pretest actuators to determine their ability to withstand the dynamic and seating torque loads which the actuator is expected to experience in field operation. While it would be desirable to repeatedly subject an actuator, under test situations, to the full and varying torque loads to be experienced in the field, current methods for testing valve actuators, such as described in standard number Qual-ACT 003 of the ADAC Division of Anchor/Darling Industries, Inc., repeatedly subject the actuators to a maximum load without simulating the full range of loads.